In a cathode-ray tube (CRT) display (e.g., television display), several hundred horizontal lines are scanned from top to bottom and from left to right. In a progressive display, each line is scanned in turn from top to bottom to constitute a frame picture. In an interlaced display, the odd-numbered lines and the even-numbered lines are separately and alternately scanned to constitute a separate field picture.
In a CRT display, for example, a frame picture for a progressive display consists of 480 raster lines and the display rate (or refresh rate) is 30 frames per second. In contrast, a field picture for an interlaced display consists of 240 raster lines and the display rate is 60 fields per second. Most televisions are interlaced display devices, and the raster lines are scanned alternately in two interwoven lines, i.e., all of the odd-numbered lines and all of the even-numbered lines.
In digital television broadcast, a digital video decoder (e.g., a set-top box) is used as a device that enables a television set to become a user interface to a communication channel and also enables a television set to receive and decode digital audio/video data. Such digital video decoder is also required to display frozen pictures as well as normal pictures in the digital television broadcast. For example, when a television viewer changes the channel, the digital video decoder of a television set operates to display a frozen picture while it performs initialization and/or satisfies conditions needed for a transition to steady-state video decode/display.
In an interlaced (or non-progressive) display, a frozen picture is displayed by alternately displaying two field pictures, one for the even-numbered lines and the other for the odd-numbered lines. In this mode, the digital video decoder operates to continuously output these two field pictures to a display unit (e.g., CRT display).
When video data is recorded as field pictures, there is a 1/60 second temporal offset between the adjacent field pictures and a corresponding spatial offset for an object in motion. In case that a frozen picture is constructed with two adjacent field pictures, any motion in the picture results in spatial difference between the two fields of the picture because they are temporally separated by an one-field display period. As a result, when a frozen picture is constructed using two adjacent fields of a picture, areas of motion in the frozen picture displayed will appear to vibrate as the two fields are alternately displayed. The higher the degree of motion, the more noticeable and bothersome the vibration.
Therefore, a need exists for a method of constructing frozen pictures which have substantially no vibration or the like in either progressive or non-progressive displays.